organisms themselves, manifested in the original discontinuity of variation."*

It is certain that more light is required upon these fundamental factors of evolution before the final word can be said upon the origin of species. That which distinguishes the species, in contrast to the variety, is the constancy of transmission of its specific characters, but it is evident that constancy here is not absolute constancy-at least it is not known to be absolute.

In variation, the nature, causes, degrees, and rate of variation are the subjects of investigation which now promise to give the true explanation of not only the nature but the origin of species.

"Materials for the Study of Variation, treated with especial regard to Discontinuity in the origin of species," by William Bateson, London, 1894, p. 567.

CHAPTER XI.

THE PRINCIPLES OF NATURAL HISTORY CLASSIFICATIONS.

ILLUSTRATED BY A STUDY OF THE CLASSIFICATION OF THE ANIMAL KINGDOM.

FOR a clear understanding of the meaning of the origin of species it is essential to consider the nature of the nomenclature of the classification of organisms We have already considered what species are and what the organic individual is, and how development is an appropriate term for the growth and perfection of the individual, and how evolution pertains to the progressive modifications of the successive species of a genus.

Classifications in Natural History.-Classifications and systems of classification in natural history are but methods of expressing, briefly, almost symbolically, the knowledge we already possess of the characters of organisms and their relations to each other. A single word, the name of a class or order, or even the specific name of a species, stands for all the morphological and physiological characters peculiar to that species, order, or class. Hence such terms are highly technical: and though it may not be possible to learn the full meaning of any of them in a brief course of lectures, it will be possible to describe the right manner of using them, so that the knowledge of the details will be arranged in an or derly manner under the proper heads as it is gradually acquired.

Species and Genus of Aristotle.-As the facts of biological science have accumulated it has been found necessary to distribute them in some systematic manner, and for this purpose a number of arbitrary divisions having definite names has been gradually evolved. The use and meaning of these names will be most easily explained by a brief examination of their development from the terms Species and Genus of Aristotelian

logic. Species, the translation of the Greek term éidos, meant, when applied to organisms, those having a number of like and peculiar characters. Genus, the translation of the Greek yevos, in logic was that which can be predicated of things differing in species, and as a biological term it was applied to a group which included several different species.

Scaliger's Terms.-Scaliger expanded the Aristotelian nomenclature by him Individual was used to indicate a single organism (plant or animal), distinguished by having a separate body, and having a separate and independent activity. Species was used in the Aristotelian sense, but Genus was found of three degrees of importance: the Genus proximum, the Genus medium, and the Genus summum.

The Terms of Linné.-Linné (1735-1766) classified organisms (both plants and animals), retained the name Genus for the Genus proximum of Scaliger, and proposed the term Ordo for Genus medium and Classis for Genus summum.

Cuvier's Perfection of the Nomenclature and the Present Usage. -These names were later adopted by Cuvier, about the beginning of the present century, and he added the term Embranchment, or Branch; and thus was established the nomenture still in use in Biology, which in English is as follows: Individual, Species, Genus, Order, Class, and Branch (or Subkingdom, or Phylum, or Type). To illustrate the meaning of these divisions the following examples may be given: A black and a bay horse would be called two individuals of the same species. The horse and the ass are two species of the same genus (Equus). A horse, an ass, and an elephant all belong to one order (Pachydermata). The horse, ass, elephant, and lion are of the same class (Mammalia). All these would be united in the same branch with the alligator (the branch Vertebrata). Further subdivision has been very commonly made of the order into suborders or families, viz., the family of Elephantida, including the elephants and the mastodon, and the family of Equide, including the horse and the Hipparion.

The Classification of Cuvier.-Linné recognized six classes in the Animal Kingdom (Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes). Cuvier made great progress in the distinc

tion of the lower animals. He recognized four branches (Animalia Vertebrata, Animalia Mollusca, Animalia Articulata, Animalia Radiata). The first four classes of Linné's system were united to form the first branch of Cuvier. The most prominent character uniting them was the possession of an internal skeleton, bound together by a segmented vertebral column. The second branch of Cuvier, called Mollusca, included six classes (Cephalopoda, Pteropoda, Gastropoda, Acephala, Brachiopoda, Cirrhopoda), and the conspicuous characters of the Mollusca were the possession of a soft, bag-like body, enclosed more or less completely by a hard exterior shell composed of one, two, or more parts. Cuvier called the third branch Articulata, including in it four classes (Annelida, Crustacea, Arachnida, Insecta). The chief character in this branch was the segmented external skeleton, composed of joints with lateral articulated appendages. The fourth branch was Radiata, and included five classes (Echinoderms, Intestinal Worms, Acalephæ, Polypi, Infusoria). The prominent character was the radiate structure, typically exhibited in the Starfish or Sea-urchin, but ignorance of internal structure led to the association of many unlike forms. Since Cuvier's time great advance has been made in the knowledge of the structural anatomy of animals, especially in the smaller and lower organisms, and many other classifications have been proposed, but the majority of Cuvier's classes have remained. Animals referred to some of the classes by Cuvier, and some newlydiscovered animals, have been made the types of other classes, and stricter definitions of the classes already established have been made.

Uniformity of Usage of Specific and Generic Names.-The branches have been considerably remodelled, especially by later zoologists, according as one or other organ or system of organs has been taken as of chief importance in distinguishing the groups. Of the later classifications those of Leuckhart, Huxley, Claus, Gegenbaur, and Lankester have expressed new points of view in the arrangement of the organisms, but in all the confusion of systems a common usage has grown up in the application of specific and generic names to animals and plants, and these have constituted the standards.

At the present time hardly two standard authors of text-books of Zoology or Paleontology will be found to apply the nomenclature of classification in the same way throughout; that is, they will not distribute the genera in the same manner, or will give different value, or will apply different names orders, families, and classes.

Selection of a Standard Classification. It becomes necessary to use some standard in the matter of classification, and Zittel's "Manual of Paleontology" may be selected as the standard in the present case. Editions of Zittel are published in both German and French, but at the present time (1895) no English edition has appeared.*

Differences of Opinion regarding the Rank of the Characters. The difference in usage of the nomenclature of classification is determined by differences of opinion as to the taxonomic value or rank of characters expressed by the organisms rather than by any difference in recognizing the characters as matters of fact. Classifications, therefore, although differing in the hands of different authors, may be used with precision when considered as descriptive of the combination of characters expressed in actual organisms.

There are several standard classifications of more or less common use among paleontologists, three of which may be here referred to: Claus and Sedgwick's, as given in "Elementary Text-book of Zoology," 1884; Zittel's classification in "Handbuch der Palæontologie," vol. I., 1876-1880; Nicholson and Lydekker, "Manual of Paleontology," 3d Ed., 1889.

Claus and Sedgwick's Definitions of the Nine Branches of the Animal Kingdom.-Brief definitions of the nine branches, as given by Claus and Sedgwick, are as follows, viz.:

"Protozoa. Of small size, with differentiations within the sarcode, without cellular organs, with predominating asexual reproduction.

"Calenterata.-Radiate animals segmented in terms of 2, 4, or 6; mesoderm of connective tissue, often gelatinous;

* A briefer text-book in German has appeared: "Grundzüge der Palæontologie (Palæozoologie)," von Karl A. von Zittel, pp. i-viii, 1-971, and 2048 figures; Munich, 1895. An English translation of this work, with some revision by American paleontologists, is in preparation.